This invention relates to a fiber optical rotation sensor, such as a fiber optical gyroscope, which measures a phase difference of light waves propagating in opposite directions within an optical fiber and senses a speed corresponding to an angle of rotation of a rotating body.
A fiber optical gyroscope, for example, as a sensor for sensing a speed corresponding to an angle of rotation is disclosed, for example, in
(1) International Publication No. WO 82/03456 entitled "Fiber Optical Rotation Sensor", published on Oct. 14, 1982;
(2) International Publication No. WO 83/01683 entitled "Multimode Fiber Optical Rotation Sensor", published on May 11, 1983; and
(3) International Publication No. WO 83/00552 entitled "Fiber Optical Rotation Sensor Utilizing Unpolarized Light" published on Feb. 17, 1983.
The fiber optical gyroscope senses an amount of a phase difference, i.e., a Sagnac phase difference, between light waves mutually counter-propagating in an optical fiber loop on a rotating body as produced due to the rotation of the rotating body. In the fiber optical gyroscope, a whole light wave propagating path located between an optical source and a sensing section for sensing interference light waves occurring due to an interference between mutually counter-propagating waves can accurately be made up of an optical fiber and thus has a longer service file.
The fiber optical gyroscope of the aforementioned International Publications uses a phase modulation system in which light waves launched into an optical fiber loop are phase-modulated for interference. In the fiber optical gyroscope the interference light wave is sensed at a photodetector and a sensed signal is supplied to a lock-in amplifier comprised of a synchronous detector and low pass filter. In the lock-in amplifier, the output signal of the photodetector is detected with a phase modulation frequency f.sub.0, only a frequency (f.sub.0) component is detected from the sensed signal by the synchronous detector and only a DC component of the detected component is delivered from the low pass filter.
The output signal of the DC component is proportional to EQU J.sub.1 (.phi.m).times.sin.DELTA..phi.
a product of Bassel function J.sub.1 (.phi.m) and sin.DELTA..phi., where
.DELTA..phi.: a Sagnac phase difference PA1 .phi.m: the phase modulation amplitude PA1 J.sub.1 (.phi.m): the Bessel function
If the amount of phase modulation is set at a stable point at which the value of a first-order Bessel function does not vary greatly, then a filter output of J.sub.1 .multidot.sin.DELTA..phi. becomes a sinusoidal function of the Sagnac phase difference .DELTA..phi. and, as shown in FIG. 1, the Sagnac phase difference .DELTA..phi. can uniquely be determined from the output level through the utilization of a DC component, i.e., a linear range I of a plot of the output level against the Sagnac phase difference .DELTA..phi. in FIG. 1.
However, this presents a narrow dynamic range problem since a measurable range of the Sagnac phase difference .DELTA..phi. is narrower at a lower level due to the use of the linear portion of sin.DELTA..phi..
In order to improve this defect, a method is proposed in Electronics Letters 10th Nov. 1983 Vol. 19 No. 23 P 997 "Direct Rotation-Rate Detection with a Fiber-Optic Gyro By Using Digital Data Processing." That is, as the output signal of the lock-in amplifier detected by a phase modulation system, the fundamental and the high harmonic wave components (2f.sub.0, 3f.sub.0, 4f.sub.0 . . . ) of a modulation frequency f.sub.0 appear, as set out below, which include a Sagnac phase difference .DELTA..phi. as a factor: ##EQU2## noting that .DELTA..phi. is proportional to the detection rate and that Jn and .phi.m represent an n-order Bessel function and phase modulation amplitude, respectively.
If, out of the output components, a ratio is taken for the 1st and 2nd harmonic waves S.sub.1 and S.sub.2 of the modulation frequency which are detected simultaneously, then a relation ##EQU3## is established. Thus the Sagnac phase difference .DELTA..phi. becomes ##EQU4## From this it will be appreciated that the Sagnac phase difference .DELTA..phi. is detected from the 1st and 2nd components S.sub.1 and S.sub.2 of the modulation frequency contained in the output signal.
In order to accurately detect the Sagnac phase difference .DELTA..phi. it is required that the factor J.sub.2 (.phi.m)/J.sub.1 (.phi.m) in Equation (3) be made constant irrespective of the phase modulation amplitude .phi.m. For the 4-th harmonic waves detected, therefore, the phase modulation amplitude should be so controlled as to make constant a ratio EQU S.sub.2 /S.sub.4 =J.sub.2 (.phi.m)/J.sub.4 (.phi.m) (4)
This method employs the ratio S.sub.1 /S.sub.2 of the two modulation frequency components and thus a light amount variation resulting from an interference noise is eliminated. It is, therefore, possible to obtain a broader dynamic range through the calculation of tan.sup.-1.
In this method, however, if as shown in FIG. 2 a better phase modulation amplitude .phi.m1 or .phi.m2 is selected for the 1st or 2nd Bessel function J.sub.1 or J.sub.2, then the variation of the factor J.sub.1 (.phi.)/J.sub.2 (.phi.m) is increased with respect to the variation of the phase modulation amplitude .phi.m. In order to achieve a better sensitivity level, it will be necessary to obtain as great an output amplitude as possible. As shown in FIG. 2, therefore, if the phase modulation amplitude .phi.m1 or .phi.m2 is set at a level at which the 1st or 2nd Bessel function J.sub.1 or J.sub.2 is maximized, then the differential coefficient J.sub.1 ' or J.sub.2 ' of the 1st or 2nd Bessel function J.sub.1 or J.sub.2 becomes greater. The variation of the phase modulation amplitude causes a greater variation in the 1st or 2nd Bessel function J.sub.2 or J.sub.1 and hence a greater variation in J.sub.1 (.phi.m)/J.sub.2 (.phi.m).
For the phase modulation amplitude .phi.m1 or .phi.m2 the component of the 4-th Bessel function J.sub.4 is at a smaller level and it is, therefore, difficult to make the phase modulation amplitude constant. As a result, J.sub.2 (.phi.m)/J.sub.1 (.phi.m) is not stabilized, thus lowering the detection amplitude level.